Creating permissive microenvironments for stem cell transplantation into the central nervous system

Opportunities and challenges: stem cell-based therapy for the treatment of ischemic stroke

Stem cell-based therapy for ischemic stroke has been widely explored in animal models and provides strong evidence of benefits. In this review, we summarize the types of stem cells, various delivery routes, and tracking tools for stem cell therapy of ischemic stroke. MSCs, EPCs, and NSCs are the most explored cell types for ischemic stroke treatment. Although the mechanisms of stem cell-based therapies are not fully understood, the most possible functions of the transplanted cells are releasing growth factors and regulating microenvironment through paracrine mechanism. Clinical application of stem cell-based therapy is still in its infancy. The next decade of stem cell research in stroke field needs to focus on combining different stem cells and different imaging modalities to fully explore the potential of this therapeutic avenue: from bench to bedside and vice versa.

Position along the nasal/temporal plane affects synaptic development by adult photoreceptors, revealed by micropatterning

In retinal degeneration, death of photoreceptors causes blindness. Repair of the retina by transplanting photoreceptors has resulted in limited functional connectivity between transplanted and host neurons. We hypothesize that absence of appropriate biological cues, specifically positional (retinotopographic) cues, reduces synaptogenesis. Here we use micropatterning to test whether regional origin affects the early synaptic development of photoreceptors. Right and left retinas from salamanders were first labelled with dextran tetramethyl-rhodamine and fluorescein, respectively, bisected into nasal (N)/temporal (T) or dorsal (D)/ventral (V) halves, individually dissociated, mixed, and cultured for 1 week. Origin of cells was identified by the fluorescent label. Interactions between photoreceptors and neighboring (target) cells were assessed by the number of neuritic contacts with a presynaptic swelling (varicosity). Randomly-plated photoreceptors showed no preference for cellular origin, likely due to multiple potential interactions available to each cell. To reduce cell-cell interactions, culture substrate was patterned using a microfluidic device with 10 μm-wide channels separated by 200 μm, thus allowing only 1-2 targets per photoreceptor. In patterned cultures, 36.89% of N rod cells contacted T targets but only 27.42% of N rod cells contacted N targets; similarly 35.05% of T rod cells contacted N cells but only 17.08% contacted T cells. Thus, opposite regions were more permissive of contact. However, neither cone nor D/V rod cells showed preferences for positional origin of targets. In conclusion, micropatterning demonstrated that neuritic differentiation by rod cells depends on retinotopographic cues along the nasal/temporal plane, suggesting that transplanting rod cells of known positional origin will increase transplant success.

Role of melatonin in traumatic brain injury and spinal cord injury

Brain and spinal cord are implicated in incidences of two of the most severe injuries of central nervous system (CNS). Traumatic brain injury (TBI) is a devastating neurological deficit involving primary and secondary injury cascades. The primary and secondary mechanisms include complex consequences of activation of proinflammatory cytokines, cerebral edema, upregulation of NF-κβ, disruption of blood-brain barrier (BBB), and oxidative stress. Spinal cord injury (SCI) includes primary and secondary injury cascades. Primary injury leads to secondary injury in which generation of free radicals and oxidative or nitrative damage play an important pathophysiological role. The indoleamine melatonin is a hormone secreted or synthesized by pineal gland in the brain which helps to regulate sleep and wake cycle. Melatonin has been shown to be a versatile hormone having antioxidative, antiapoptotic, neuroprotective, and anti-inflammatory properties. It has a special characteristic of crossing BBB. Melatonin has neuroprotective role in the injured part of the CNS after TBI and SCI. A number of studies have successfully shown its therapeutic value as a neuroprotective agent in the treatment of neurodegenerative diseases. Here in this review we have compiled the literature supporting consequences of CNS injuries, TBI and SCI, and the protective role of melatonin in it.

3D culture of murine neural stem cells on decellularized mouse brain sections

Transplantation of neural stem cells (NSC) in diseased or injured brain tissue is widely studied as a potential treatment for various neurological pathologies. However, effective cell replacement therapy relies on the intrinsic capacity of cellular grafts to overcome hypoxic and/or immunological barriers after transplantation. In this context, it is hypothesized that structural support for grafted NSC will be of utmost importance. With this study, we present a novel decellularization protocol for 1.5 mm thick mouse brain sections, resulting in the generation of acellular three-dimensional (3D) brain sections. Next, the obtained 3D brain sections were seeded with murine NSC expressing both the eGFP and luciferase reporter proteins (NSC-eGFP/Luc). Using real-time bioluminescence imaging, the survival and growth of seeded NSC-eGFP/Luc cells was longitudinally monitored for 1-7 weeks in culture, indicating the ability of the acellular brain sections to support sustained ex vivo growth of NSC. Next, the organization of a 3D maze-like cellular structure was examined using confocal microscopy. Moreover, under mitogenic stimuli (EGF and hFGF-2), most cells in this 3D culture retained their NSC phenotype. Concluding, we here present a novel protocol for decellularization of mouse brain sections, which subsequently support long-term 3D culture of undifferentiated NSC.

Engineered N-cadherin and L1 biomimetic substrates concertedly promote neuronal differentiation, neurite extension and neuroprotection of human neural stem cells

We investigated the design of neurotrophic biomaterial constructs for human neural stem cells, guided by neural developmental cues of N-cadherin and L1 adhesion molecules. Polymer substrates fabricated either as two-dimensional (2-D) films or three-dimensional (3-D) microfibrous scaffolds were functionalized with fusion chimeras of N-cadherin-Fc alone and in combination with L1-Fc, and the effects on differentiation, neurite extension and survival of H9 human-embryonic-stem-cell-derived neural stem cells (H9-NSCs) were quantified. Combinations of N-cadherin and L1-Fc co-operatively enhanced neuronal differentiation profiles, indicating the critical nature of the two complementary developmental cues. Notably, substrates presenting low levels of N-cadherin-Fc concentrations, combined with proportionately higher L1-Fc concentration, most enhanced neurite outgrowth and the degree of MAP2+ and neurofilament-M+ H9-NSCs. Low N-cadherin-Fc alone promoted improved cell survival following oxidative stress, compared to higher concentrations of N-cadherin-Fc alone or combinations with L1-Fc. Pharmacological and antibody blockage studies revealed that substrates presenting low levels of N-cadherin are functionally competent so long as they elicit a threshold signal mediated by homophilic N-cadherin and fibroblast growth factor signaling. Overall, these studies highlight the ability of optimal combinations of N-cadherin and L1 to recapitulate a "neurotrophic" microenvironment that enhances human neural stem cell differentiation and neurite outgrowth. Additionally, 3-D fibrous scaffolds presenting low N-cadherin-Fc further enhanced the survival of H9-NSCs compared to equivalent 2-D films. This indicates that similar biofunctionalization approaches based on N-cadherin and L1 can be translated to 3-D "transplantable" scaffolds with enhanced neurotrophic behaviors. Thus, the insights from this study have fundamental and translational impacts for neural-stem-cell-based regenerative medicine.

Stem cell therapy for central nerve system injuries: glial cells hold the key

Mammalian adult central nerve system (CNS) injuries are devastating because of the intrinsic difficulties for effective neuronal regeneration. The greatest problem to be overcome for CNS recovery is the poor regeneration of neurons and myelin-forming cells, oligodendrocytes. Endogenous neural progenitors and transplanted exogenous neuronal stem cells can be the source for neuronal regeneration. However, because of the harsh local microenvironment, they usually have very low efficacy for functional neural regeneration which cannot compensate for the loss of neurons and oligodendrocytes. Glial cells (including astrocytes, microglia, oligodendrocytes and NG2 glia) are the majority of cells in CNS that provide support and protection for neurons. Inside the local microenvironment, glial cells largely influence local and transplanted neural stem cells survival and fates. This review critically analyzes current finding of the roles of glial cells in CNS regeneration, and highlights strategies for regulating glial cells’ behavior to create a permissive microenvironment for neuronal stem cells.

MicroRNA-34a Modulates Neural Stem Cell Differentiation by Regulating Expression of Synaptic and Autophagic Proteins

We have previously demonstrated the involvement of specific apoptosis-associated microRNAs (miRNAs), including miR-34a, in mouse neural stem cell (NSC) differentiation. In addition, a growing body of evidence points to a critical role for autophagy during neuronal differentiation, as a response-survival mechanism to limit oxidative stress and regulate synaptogenesis associated with this process. The aim of this study was to further investigate the precise role of miR-34a during NSC differentiation. Our results showed that miR-34a expression was markedly downregulated during neurogenesis. Neuronal differentiation and cell morphology, synapse function, and electrophysiological maturation were significantly impaired in miR-34a-overexpressing NSCs. In addition, synaptotagmin 1 (Syt1) and autophagy-related 9a (Atg9a) significantly increased during neurogenesis. Pharmacological inhibition of autophagy impaired both neuronal differentiation and cell morphology. Notably, we showed that Syt1 and Atg9a are miR-34a targets in neural differentiation context, markedly decreasing after miR-34a overexpression. Syt1 overexpression and rapamycin-induced autophagy partially rescued the impairment of neuronal differentiation by miR-34a. In conclusion, our results demonstrate a novel role for miR-34a regulation of NSC differentiation, where miR-34a downregulation and subsequent increase of Syt1 and Atg9a appear to be crucial for neurogenesis progression.

Microglia centered pathogenesis in ALS: insights in cell interconnectivity

Amyotrophic lateral sclerosis (ALS) is the most common and most aggressive form of adult motor neuron (MN) degeneration. The cause of the disease is still unknown, but some protein mutations have been linked to the pathological process. Loss of upper and lower MNs results in progressive muscle paralysis and ultimately death due to respiratory failure. Although initially thought to derive from the selective loss of MNs, the pathogenic concept of non-cell-autonomous disease has come to the forefront for the contribution of glial cells in ALS, in particular microglia. Recent studies suggest that microglia may have a protective effect on MN in an early stage. Conversely, activated microglia contribute and enhance MN death by secreting neurotoxic factors, and impaired microglial function at the end-stage may instead accelerate disease progression. However, the nature of microglial–neuronal interactions that lead to MN degeneration remains elusive. We review the contribution of the neurodegenerative network in ALS pathology, with a special focus on each glial cell type from data obtained in the transgenic SOD1G93A rodents, the most widely used model. We further discuss the diverse roles of neuroinflammation and microglia phenotypes in the modulation of ALS pathology. We provide information on the processes associated with dysfunctional cell–cell communication and summarize findings on pathological cross-talk between neurons and astroglia, and neurons and microglia, as well as on the spread of pathogenic factors. We also highlight the relevance of neurovascular disruption and exosome trafficking to ALS pathology. The harmful and beneficial influences of NG2 cells, oligodendrocytes and Schwann cells will be discussed as well. Insights into the complex intercellular perturbations underlying ALS, including target identification, will enhance our efforts to develop effective therapeutic approaches for preventing or reversing symptomatic progression of this devastating disease.

Application of cell encapsulation for controlled delivery of biological therapeutics

Cell microencapsulation technology is likely to have an increasingly important role in new approaches rather than the classical and pioneering organ replacement. Apart from becoming a tool for protein and morphogen release and long-term drug delivery, it is becoming a new three-dimensional platform for stem cell research. Recent progress in the field has resulted in biodegradable scaffolds that are able to retain and release the cell content in different anatomical locations. Additional advances include the use biomimetic scaffolds that provide greater control over material-cell interactions and the development of more precise encapsulated cell-tracking systems. This review summarises the state of the art of cell microencapsulation and discusses the main directions and challenges of this field towards the controlled delivery of biological therapeutics.

RhoC involved in the migration of neural stem/progenitor cells

Alzheimer's disease (AD) is characterized by deposition of beta-amyloid peptides (Aβ) and progressive loss of neurons. Neural stem/progenitor cells (NSPCs) can proliferate and produce immature neurons even in the brain of AD patients. However, Aβ42 significantly decreased the expression of RhoC in NSPCs during the co-incubation (P < 0.01). Treating with RhoC siRNA prevented membrane from protrusion and led to a significant reduction in cell migration in responses to SDF-1. Compared with wild-type mice, the numbers of RhoC-immunoreactive cells in hippocampus and cortex were significantly down-regulated in APP/PS1 mice aged 9 months. The results suggest that Aβ42 down-regulates the expression of RhoC in NSPCs in vitro and in vivo; down-regulated RhoC expression results in decreased migration of NSPCs.

Neural stem cell transplantation in an animal model of traumatic brain injury

The central nervous system (CNS) can be damaged by a wide range of conditions resulting in loss of specific populations of neurons and/or glial cells and in the development of defined psychiatric or neurological symptoms of varying severity. As the CNS has limited inherent capacity to regenerate lost tissue and self-repair, the development of therapeutic strategies for the treatment of CNS insults remains a serious scientific challenge with potential important clinical applications. In this context, strategies involving transplantation of specific cell populations, such as stem cells and neural stem cells (NSCs), to replace damaged cells offers an opportunity for the development of cell-based therapies. Along these lines, in this review we describe a protocol which involves transplantation of NPCs, genetically engineered to overexpress the neurogenic molecule Cend1 and have thus the potency to differentiate with higher frequency towards the neuronal lineage in a rodent model of stab wound cortical injury.

Thiol‐X Reactions in Tissue Engineering

Thiol‐based click reactions have played a key role in the synthesis of biomaterial scaffolds for regenerative medicine applications. Of particular importance has been their use in creating cell‐laden hydrogel matrices for both fundamental and translational applications. Thiol‐X reactions are often exploited in biological applications as they allow for the facile incorporation of biofunctional components, which has led to many key advancements for the field of tissue engineering. In this chapter, we summarize the important considerations for cytocompatible macromolecular monomer design and subsequent cellular encapsulation in hydrogel formulations. Briefly, we review the main thiol‐X reactions that have been used to synthesize hydrogel cell scaffold systems; provide a generalized protocol for the preparation of cell‐laden hydrogels; present highlights that demonstrate specific advantages of thiol‐X reactions and advances in their application in regenerative medicine research; and conclude with a prospectus on future directions for the field in using thiol‐X chemistries to engineer more advanced hydrogel materials.

Biomaterials for spinal cord repair

Spinal cord injury (SCI) results in permanent loss of function leading to often devastating personal, economic and social problems. A contributing factor to the permanence of SCI is that damaged axons do not regenerate, which prevents the re-establishment of axonal circuits involved in function. Many groups are working to develop treatments that address the lack of axon regeneration after SCI. The emergence of biomaterials for regeneration and increased collaboration between engineers, basic and translational scientists, and clinicians hold promise for the development of effective therapies for SCI. A plethora of biomaterials is available and has been tested in various models of SCI. Considering the clinical relevance of contusion injuries, we primarily focus on polymers that meet the specific criteria for addressing this type of injury. Biomaterials may provide structural support and/or serve as a delivery vehicle for factors to arrest growth inhibition and promote axonal growth. Designing materials to address the specific needs of the damaged central nervous system is crucial and possible with current technology. Here, we review the most prominent materials, their optimal characteristics, and their potential roles in repairing and regenerating damaged axons following SCi.

A microfluidic chip using phenol formaldehyde resin for uniform-sized polycaprolactone and chitosan microparticle generation

This study develops a new solvent-compatible microfluidic chip based on phenol formaldehyde resin (PFR). In addition to its solvent-resistant characteristics, this microfluidic platform also features easy fabrication, organization, decomposition for cleaning, and reusability compared with conventional chips. Both solvent-dependent (e.g., polycaprolactone) and nonsolvent-dependent (e.g., chitosan) microparticles were successfully prepared. The size of emulsion droplets could be easily adjusted by tuning the flow rates of the dispersed/continuous phases. After evaporation, polycaprolactone microparticles ranging from 29.3 to 62.7 μm and chitosan microparticles ranging from 215.5 to 566.3 μm were obtained with a 10% relative standard deviation in size. The proposed PFR microfluidic platform has the advantages of active control of the particle size with a narrow size distribution as well as a simple and low cost process with a high throughput.

Promotion of neuronal differentiation of neural progenitor cells by using EGFR antibody functionalized collagen scaffolds for spinal cord injury repair

The main challenge for neural progenitor cell (NPC)-mediated repair of spinal cord injury (SCI) is lack of favorable environment to direct its differentiation towards neurons rather than glial cells. The myelin associated inhibitors have been demonstrated to promote NPC differentiation into glial lineage. Herein, to inhibit the downstream signaling activated by myelin associated inhibitors, cetuximab, an epidermal growth factor receptor (EGFR) neutralizing antibody, functionalized collagen scaffold has been developed as a vehicle for NPC implantation. It was found that collagen-cetuximab 1 μg scaffolds enhanced neuronal differentiation and inhibited astrocytic differentiation of NPCs exposed to myelin proteins significantly in vitro. To test the therapeutic effect in vivo, NPCs expressing green fluorescent protein (GFP)-embedded scaffolds have been implanted into the 4 mm-long hemisection lesion of rats. We found that the collagen-cetuximab 5 μg scaffolds induced neuronal differentiation and decreased astrocytic differentiation of NPCs, enhanced axon regeneration, and promoted functional recovery markedly. A well-functionalized scaffold was constructed to improve the recovery of SCI, which could promote the neuronal differentiation of neural progenitor cells in vivo.

Spatiotemporal evolution of early innate immune responses triggered by neural stem cell grafting

Introduction

Transplantation of neural stem cells (NSCs) is increasingly suggested to become part of future therapeutic approaches to improve functional outcome of various central nervous system disorders. However, recently it has become clear that only a small fraction of grafted NSCs display long-term survival in the (injured) adult mouse brain. Given the clinical invasiveness of NSC grafting into brain tissue, profound characterisation and understanding of early post-transplantation events is imperative to claim safety and efficacy of cell-based interventions.

Methods

Here, we applied in vivo bioluminescence imaging (BLI) and post-mortem quantitative histological analysis to determine the localisation and survival of grafted NSCs at early time points post-transplantation.

Results

An initial dramatic cell loss (up to 80% of grafted cells) due to apoptosis could be observed within the first 24 hours post-implantation, coinciding with a highly hypoxic NSC graft environment. Subsequently, strong spatiotemporal microglial and astroglial cell responses were initiated, which stabilised by day 5 post-implantation and remained present during the whole observation period. Moreover, the increase in astrocyte density was associated with a high degree of astroglial scarring within and surrounding the graft site. During the two-week follow up in this study, the NSC graft site underwent extensive remodelling with NSC graft survival further declining to around 1% of the initial number of grafted cells.

Conclusions

The present study quantitatively describes the early post-transplantation events following NSC grafting in the adult mouse brain and warrants that such intervention is directly associated with a high degree of cell loss, subsequently followed by strong glial cell responses.

Ephrin-B3 decreases the survival of adult rat spinal cord-derived neural stem/progenitor cells in vitro and after transplantation into the injured rat spinal cord

Although transplantation of neural stem/progenitor cells (NSPC) encourages regeneration and repair after spinal cord injury (SCI), the survival of transplanted NSPC is limited. Ephrin-B3 has been shown to reduce the death of endogenous NSPC in the subventricular zone of the mouse brain without inducing uncontrolled proliferation. Due to similarities in the environment of the brain and spinal cord, we hypothesized that ephrin-B3 might reduce the death of both transplanted and endogenous spinal cord-derived NSPC. Both normal and injured (26 g clip compression) spinal cords were examined. Ephrin-B3-Fc was tested, and Fc fragments and phosphate-buffered saline (PBS) were used as controls. We found that EphA4 receptors were expressed by spinal cord-derived NSPC and expressed in the normal and injured rat spinal cord (higher expression in the latter). In vitro, ephrin-B3-Fc did not significantly reduce the survival of NSPC except at 1 μg/mL (P<0.05), but Fc fragments alone reduced NSPC survival at all doses in a dose-dependent fashion. In vivo, intrathecal infusion of ephrin-B3-Fc increased the proliferation of endogenous ependymal cells and the proportion of proliferating cells that expressed the glial fibrillary acidic protein astrocytic marker in the injured spinal cord compared with the infusion of PBS (P<0.05). However, in the injured spinal cord, the infusion of either ephrin-B3-Fc or Fc fragments alone caused a 20-fold reduction in the survival of transplanted NSPC (P<0.001). Thus, after SCI, ephrin-B3-Fc and Fc fragments are toxic to transplanted NSPC.

Engineering therapies in the CNS: what works and what can be translated

Engineering is the art of taking what we know and using it to solve problems. As engineers, we build tool chests of approaches; we attempt to learn as much as possible about the problem at hand, and then we design, build, and test our approaches to see how they impact the system. The challenge of applying this approach to the central nervous system (CNS) is that we often do not know the details of what is needed from the biological side. New therapeutic options for treating the CNS range from new biomaterials to make scaffolds, to novel drug-delivery techniques, to functional electrical stimulation. However, the reality is that translating these new therapies and making them widely available to patients requires collaborations between scientists, engineers, clinicians, and patients to have the greatest chance of success. Here we discuss a variety of new treatment strategies and explore the pragmatic challenges involved with engineering therapies in the CNS.

Non-invasive imaging of transplanted human neural stem cells and ECM scaffold remodeling in the stroke-damaged rat brain by (19)F- and diffusion-MRI

Transplantation of human neural stem cells (hNSCs) is emerging as a viable treatment for stroke related brain injury. However, intraparenchymal grafts do not regenerate lost tissue, but rather integrate into the host parenchyma without significantly affecting the lesion cavity. Providing a structural support for the delivered cells appears important for cell based therapeutic approaches. The non-invasive monitoring of therapeutic methods would provide valuable information regarding therapeutic strategies but remains a challenge. Labeling transplanted cells with metal-based (1)H-magnetic resonance imaging (MRI) contrast agents affects the visualization of the lesion cavity. Herein, we demonstrate that a (19)F-MRI contrast agent can adequately monitor the distribution of transplanted cells, whilst allowing an evaluation of the lesion cavity and the formation of new tissue on (1)H-MRI scans. Twenty percent of cells labeled with the (19)F agent were of host origin, potentially reflecting the re-uptake of label from dead transplanted cells. Both T(2)- and diffusion-weighted MRI scans indicated that transplantation of hNSCs suspended in a gel form of a xenogeneic extracellular matrix (ECM) bioscaffold resulted in uniformly distributed cells throughout the lesion cavity. However, diffusion MRI indicated that the injected materials did not yet establish diffusion barriers (i.e. cellular network, fiber tracts) normally found within striatal tissue. The ECM bioscaffold therefore provides an important support to hNSCs for the creation of de novo tissue and multi-nuclei MRI represents an adept method for the visualization of some aspects of this process. However, significant developments of both the transplantation paradigm, as well as regenerative imaging, are required to successfully create new tissue in the lesion cavity and to monitor this process non-invasively.